Present-day mobile telephony systems are normally intended for outside use. Although mobile telephones for use in such systems can often also be used indoors, the traffic capacity is greatly restricted, because the number of available radio channels for outside use is not sufficient to meet the demand of a large number of subscribers who rotate relatively close to one another, such as in an office building, for instance.
There are special mobile telephony systems for indoor use, such as inside office buildings and factories. Indoor systems, however, are assigned a different frequency band to systems that are intended for outdoor use and operate according to another standard, for instance according DECT instead of, for instance GSM. Consequently, a mobile telephone intended for an outdoor system cannot be used in an indoor system, and vice versa, which is a disadvantage.
There is proposed in Swedish Patent Application No. 9001312-9 a mobile telephony system which is intended for both outdoor and indoor use. The system includes an external part-system and an internal part-system. Adaptive channel allocation is applied in the internal part-system, therewith providing an indoor user access to all of the channels in the system. The transmitter powers are small in comparison with the transmitter powers in the external part-system, which makes it possible to use the radio channels in several cells within a building. There is, however, a serious risk that indoor connections will be disturbed by radio traffic from outdoor connections, so as to impair speech quality. A change of channel, so-called handoff, takes place when disturbances exceed a given level. The connection is broken when no free channel or non-disturbed channel is found. Thus, radio disturbances, which may also derive from other indoor connections, impair speech quality and limit the maximum traffic capacity in the internal part-system.
Furthermore, other types of electronic equipment than equipment for mobile telephony systems sensitive to disturbances from mobile telephones are found in some areas. For instance, medical apparatus in hospitals and control logic equipment in aircraft have been found to be disturbed by mobile telephones.
It is also found that the mobile telephone will not function when subjected to electrical disturbances generated in the vicinity of the mobile telephone. This can occur, for instance, in factories or plants in which certain electrical apparatus are installed.
It is a natural desire to obtain a mobile telephony system which can be used by outdoor subscribers, for instance in automotive vehicles while, at the same time, affording a high traffic capacity and a relatively disturbance-free speech transmission for indoor subscribers, for instance in buildings in which a large number of subscribers are located relatively close to one another. It is also desirable that the system can be used by subscribers who are present in locations where the surroundings contain disturbance-sensitive electronic apparatus of a kind different to that used in mobile telephony equipment, and in locations where ambient radio disturbances occur for reasons other than the presence of mobile telephony equipment.